Microelectronic device assemblies, such as memory chips and microprocessor chips, typically include one or more microelectronic components attached to a substrate and encased in a protecting covering. The microelectronic components commonly include at least one microelectronic die having functional features such as memory cells, processor circuits, and interconnecting circuitry. The dies also typically include bond-pads electrically coupled to the functional features. The bond-pads can be used to operatively connect the dies to external devices such as buses, circuits, and/or other microelectronic assemblies.
A plurality of microelectronic dies are generally formed simultaneously in a single microfeature workpiece or wafer. The dies typically have an active side with bond-pads that initially face upward. One step in the manufacturing process is the formation of conductive couplers (e.g., solder balls or pads of solder paste) on the bond-pads. For example, after forming the dies on the wafer, a highly accurate stenciling machine can deposit masses of solder paste onto the individual pads on the dies to form solder balls.
The stenciling machine generally includes a stencil and a wiper mechanism. In applications where the bond-pads on the dies have a very fine pitch, however, patterned layers of photoresists are typically used rather than stencils. In fine pitch applications, a resist is applied to the wafer and patterned to form a plurality of holes arranged in a pattern corresponding to the bond-pads on the dies. A wiper mechanism is then moved across the resist to drive the solder paste through the holes and into contact with the bond-pads on the wafer. The resist is then stripped away and the wafer is ready for further processing. One drawback associated with this method is that it includes a number of relatively expensive steps. For example, manufacturers must strip the resist and dispose of the chemical waste generated during removal of the resist. This can be quite expensive because there are many regulations for disposing of such chemical wastes. Another drawback with this method is that removing the resist may require chemical solvents that can attack (e.g., contaminate and/or damage) the various components of the dies and/or the wafer.
Another step in the packaging process is dicing or singulating the dies from the wafer and attaching the singulated dies to external devices. One type of microelectronic component, for example, is a “flip-chip” device. These components are referred to as “flip-chips” because after forming the solder balls on the bond-pads and singulating the dies, the individual dies are inverted or “flipped” such that the bond-pads face downward for attachment to terminals of a lead frame or interposer substrate. In applications using solder bumps, the solder bumps are reflowed to form a solder joint between the flip-chip component and the substrate. This leaves a small gap between the flip-chip and the substrate. To enhance the integrity of the joint between the microelectronic component and the substrate, an underfill material is introduced into the gap.
There are several drawbacks associated with this method of applying the underfill material. For example, the underfill material is typically dispensed into the gap by depositing a bead of the underfill material along one or two sides of the flip-chip when the underfill material is in a fluidic state (i.e., flowable) and allowing the underfill material to wick into the gap. After the underfill material fills the gap, it is cured to a hardened state. Although such a process yields good results, the processing time necessary to permit the underfill material to flow across the entire width of the die can reduce the throughput of the manufacturing process. Moreover, depositing and curing the underfill material necessitates further steps in the packaging process that can decrease throughput. Yet another drawback with this above process for depositing the underfill material is that one side of the flip-chip often has a greater concentration of underfill material. The nonuniform distribution of underfill material creates differences in the rigidity and the coefficient of thermal expansion across the die. Accordingly, new methods are needed for both forming stencils in fine pitch applications as well as applying underfill materials in flip-chip devices.